Screening and evaluation of potential fungal antagonists for the biological control of Fusarium head blight incited by Gibberella zeae

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Inch, Sharon Anne
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Fusarium head blight (FHB) is an important disease of wheat and other small grain cereals. The principal pathogen associated with FHB in Manitoba is Gibberella zeae (Schwein.) Petch (anamorph = Fusarium graminearum Schwabe). Currently there no biological control agents registered for the control of FHB. The overall objects of the project were to identify isolates that may be used in the biological control of FHB and to investigate the interaction between the identified biocontrol agents and G. zeae.In this study, 150 bacteria and 29 fungi were isolated from soil, wheat heads and crop debris from southern Manitoba. An additional 10 isolates of Trichoderma harzianum were obtained from the Canadian Collection of Fungal Cultures, CCFC (Ottawa, Ontario). The T. harzianum isolate, T-22 (RootShield) was included as a positive control. All were screened for inhibition of Gibberella zeae using confrontation plate assays in vitro and seed, wheat head, and straw assays in planta. Only 6% of bacterial and 45% of fungal isolates tested in the confrontation plate assays. Of the 6% of the bacterial isolates none significantly reduced FHB disease on wheat heads or reduced perithecial production on wheat straw, and had a negative effect on seed germination, therefore were not further evaluated. Chrysosporium sp. and Penicillium spp. and Trichoderma harzianum, were the fungal species that inhibited the growth of G. zeae by more than 50%. Of which Trichoderma isolates were the most effective and were able to over-grow G. zeae. Fourteen of the 18 isolates tested, including six Trichoderma and two Chrysosporium isolates, significantly reduced perithecial production on wheat straw by 52-89% compared to the control. From this study, Trichoderma harzianum was identified as most potentially effective candidate for the biocontrol of Gibberella zeae. Spore suspensions and cell-free filtrates of Trichoderma harzianum isolates were evaluated for their effectiveness in reducing perithecial and ascospore production of Gibberella zeae on wheat straw. Five T. harzianum isolates, including T-22 (RootShieldTM), reduced perithecial formation by 70% or more. Perithecial reduction was highest (96-99%) when T. harzianum spore suspension or cell-free filtrate was applied to straw 24 hours prior to inoculation with G. zeae. Control was less effective when T. harzianum was applied at the same time (co-inoculated) or 24 hours after G. zeae. Field trials showed significant reduction of perithecia on residues treated with T. harzianum prior to placement on the soil surface. Identification of those compounds in the cell-free filtrate most likely to affect biocontrol was accomplished through the use of cluster analysis, ordination and regression methods. It was found that isolates that produce similar levels of biocontrol had similar chemical composition. Ultrastructural changes were observed primarily in the exterior cells of the outer cell wall. Cytoplasmic degradation, invagination of the plasma cell membrane and thin cell walls were observed in the treated samples. Immature perithecia were overgrown by T. harzianum 15 days after co-inoculation. Few perithecia were overgrown at later stages. The perithecia affected by T. harzianum collapsed 21 days after inoculation (dai), compared to the perithecia in the untreated samples which collapsed 28 dai.
biocontrol, Tricoderma, Gibberella, Fusarium